The present invention relates to a process for preparing 2,6-dimethylphenol, and more particularly to a process for preparing 2,6-dimethylphenol from 2,4,6-trimethylphenol.
Poly(2,6-dimethyl-1,4-phenylene oxide), abbreviated as PPO, is one of the leading engineering polymers. PPO blends with polystyrene or polyamide (known as modified PPO or MPPO) are widely used in the formation of cases of personal computers or their peripheral equipment, the parts used in vehicles, electronic or electrical equipment, and house or office appliances.
2,6-dimethylphenol is the starting material for the manufacture of PPO. Typical industrial processes for preparing 2,6-dimethylphenol involve the reaction of phenol and methanol in the presence of a metal oxide catalyst. The major by-product of this reaction is 2,4,6-trimethylphenol, which has little practical use. It is desirable to convert 2,4,6-trimethylphenol into 2,6-dimethylphenol to improve the process economic benefit. The known methods for converting 2,4,6-trimethylphenol into 2,6-dimethylphenol include a catalytic dealkylation process and a catalytic hydrodealkylation process to remove the methyl group in the 4th position of 2,4,6-trimethylphenol (described in U.S. Pat. Nos. 4,533,767, 4,533,768, 4,560,810 and 4,929,766 ). However, the 2,6-dimethylphenol selectivities obtained from these dealkylation methods are rather low.
Another method for the converting of 2,4,6-trimethylphenol to 2,6-dimethylphenol is discolsed in U.S. Pat. No. 5,475,156. The process comprises the selective oxidation of 2,4,6-trimethylphenol to 3,5-dimethyl-4-hydroxybenzaldehyde in the presence of a copper-based catalyst, and the deformylation of the resulting 3,5-dimethyl-4-hydroxybenzaldehyde to 2,6-dimethylphenol in the presence of a palladium catalyst. However, the process has the following disadvantages: (1) significant amounts of undesirable by-product, 2,6-dimethyl-p-benzoquinone, are produced from the selective oxidation catalyzed by the copper-based catalysts, (2) the palladium catalyst used for catalyzing the deformylation reaction is very expensive.
The present invention provides an improved process for preparing 2,6-dimethylphenol to overcome the problems described above.
It is an object of the present invention to provide a process for preparing 2,6-dimethylphenol from 2,4,6-trimethylphenol.
In accordance with one aspect of the present invention, the process for preparing 2,6-dimethylphenol from a 2,4,6-trimethylphenol solution includes steps of (1) effecting a selective oxidation of the 2,4,6-trimethylphenol solution by reacting with an oxygen-containing gas to produce 3,5-dimethyl-4-hydroxybenzaldehyde at temperatures of 20 to 200xc2x0 C. in the presence of an iron-containing catalyst system and a first solvent, and (2) effecting a deformylation of the 3,5-dimethyl-4-hydroxybenzaldehyde to 2,6-dimethylphenol at temperatures of 50 to 400xc2x0 C. in the presence of a copper-containing catalyst system and a second solvent.
Preferably, the first solvent used for the selective oxidation is an alcohol having 1 to 6 carbon atoms. More preferably, the first solvent is methanol.
Preferably, the iron-containing catalyst system used for the selective oxidation includes an iron-containing material and a carrier. The iron-containing material is preferably one selected from a group consisting of iron halide, iron oxide, iron sulfide, iron sulfate, iron carbonate, iron hydroxide, iron complex, iron ion, iron metal, organic iron compound and the combination thereof. More preferably, the iron-containing material is iron halide. Much more preferably, the iron-containing material is iron chloride. The carrier of the iron-containing catalyst system is used for supporting the iron-containing material and selected from a group consisting of alumina, silica, titanium dioxide, zirconium dioxide, alumina-silica and zeolite.
Preferably, the iron-containing catalyst system further includes a nitrogen-containing organic compound as promoter. The nitrogen-containing organic compound is preferably an oxime, and more preferably an acetone oxime.
Preferably, the second solvent used for the deformylation is an alkane having 10 to 20 carbon atoms.
Preferably, the copper-containing catalyst system used for the deformylation includes a copper-containing material and a carrier. The copper-containing material is preferably one selected from a group consisting of copper halide, copper oxide, copper sulfide, copper sulfate, copper lactate, copper hydroxide, copper nitrate, copper oleate, copper phosphate, copper phosphoric, copper complex, copper ion, copper metal, organic copper compound and the combination thereof. More preferably, the copper-containing material is copper halide. The carrier of the copper-containing catalyst system is used for supporting the copper-containing material and selected from a group consisting of alumina, silica, titanium dioxide, zirconium dioxide, alumina-silica and zeolite.
It is another object of the present invention to provide a process for improving the selectivity of 3,5-dimethyl-4-hydroxybenzaldehyde from a 2,4,6-trimethylphenol solution. The process is carried out by reacting a the 2,4,6-trimethylphenol solution with an oxygen-containing gas to produce 3,5-dimethyl-4-hydroxybenzaldehyde at temperatures of 20 to 200xc2x0 C. in the presence of an iron-containing catalyst system and a solvent.
It is another object of the present invention to provide a process for preparing 2,6-dimethylphenol form 3,5-dimethyl-4-hydroxybenzaldehyde by using a cheaper catalyst in place of the palladium catalyst. The process is carried out by performing a deformylation of the 3,5-dimethyl-4-hydroxybenzaldehyde to the 2,6-dimethylphenol at temperatures of 50 to 400xc2x0 C. in the presence of a copper-containing catalyst system and a solvent.